Enhancement of electrostatic images



p 9, 1970 B. KAZAN 3,531,646

' ENHANCEMENT OF ELECTROSTATIC IMAGES Filed Sept. 29;l966 2 Sheets-Sheet l FIELD-EFFECT SEMICONDUCTOR 7ELECTROLUMINESCENT FIG. I

. 24 2 25 ELECTROLUMINESCENT 1 4 f FIELD-EFFECT SEMICONDUCTOR FIG. 2

30 3F-FIELD-EFFECT SEMICONDUCTOR l3 32\ 9 PHOTOCONOUCTOR FIG. 5

V INVENTOR. BENJAMIN KAZAN A ORNEY P 9, 1970 B. KAZAN 3,531,646

ENHANCEMENT OF ELECTROSTATIC IMAGES Filed Sept. 29. 1966 2 SheetsShet 2 6 l2 8 MA P m \CR FIG. 3

FIELD-EFFECT SEMICONDUCTOR /7 I'IIIIIIIII IIIIII I'III llllllllrlllllllln IIIII//III:IIIIIIIIIIIl 38 45 40 (ELECTROLUMINESCENT INVENTOR. BENJAMIN KAZAN A T TORNEV United States Patent 01 ice 3,531,646 Patented Sept. 29, 1970 3 531 646 ENHANCEMENT oFELiscTRosTAnc IMAGES Benjamin Kazan, Pasadena, Calif., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Sept. 29, 1966, Ser. No. 582,858 Int. Cl. H011 17/00; G03g /00 US. Cl. (l213 15 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to the art of xerography and more specifically to methodology useful in augmentation of latent electrostatic images.

Electrophotographic processes are invariably characterized by the presence at one point in the practice thereof, of an electrostatic latent image, which image may or may not be subsequentially developed to render a visually discernible counterpart. The formation of the electrostatic latent image may thus be regarded as the sine qua non of xerographic processes, and enormous amounts of research effort have accordingly been expended in improving both electrostatic imaging systems and the methodology utilized therewith. While great progress has resulted from such efforts, in one respect at least, electrophotography has lagged far behind the more conventional silver haloid photographic technology: reference here is made to the all too well known fact that the light response achievable with Xerographic imaging systems is of a low order of magnitude in comparison to that possible with light-responsive chemical emulsions.

While various approaches have been taken in the past in an attempt to one way or another effectively increase the response of xerographic receptors to a point where low level light recording becomes practical, it is unfortunately true that such prior attempts have to the present time met with but limited success. Although numerous reasons may be assigned to explain the limited success evidenced in the prior technology, it is evident that a major road block has occurred because researchers in general have attempted to increase the response of the receptor to light rather than attempted to operate upon the faint latent image itself to bring it up to an acceptable utilization level.

In accordance with the foregoing it is an object of the present invention to effectively increase the light utilization levels of xerographic materials by providing a method for augmenting latent electrostatic images.

It is a further object of the present invention to provide a method by the practice of which the small charge variations present in a faint electrostatic image may be amplitied to a point where further utilization of the image becomes practical.

It is an additional object of the present invention to provide a latent image enhancement method by the practice whereof common zinc oxide-coated xerographic materials may be utilized at light exposure levels far below those formerly deemed necessary.

Now in accordance with the present invention I have discovered that the latent electrostatic imagewhen formed upon suitable photoreceptors-may itself be utilized in a unique feed-back process to augment the charge variations present in the image. The discovery derives from the observation that a latent charge pattern can be formed contiguous to semiconductor materials of a type displaying, by field-effect action, varying conductivity levels in response to the electric field associated with the charge pattern. Such a phenomenon arises, for example, where a latent charge image is formed on common zinc oxide-coated paper, as the Zinc oxide itself happens to be a semiconductor of the type alluded to and responds to a latent charge image on the surface thereof, by displaying conductivity variations of the type indicated. According to the present inventive method, a latent image is initially formed on a photoconductor surface contiguous to such a semiconductor. Alternatively the semiconductor may itself comprise an effective photoconductor in which case the photoconductor surface directly bounds the semiconductor. The image-bearing surface and abounding semiconductor layer are then brought into contact with an electroluminescent layer and means are provided to establish an'electric field across portions of the electroluminescent layer and the sandwiched semiconductor. The electric field is selectively intensified and in some instances deflected by the presence in the semiconductor layer of volumes of material which-in accordance with the foregoing pattern of the latent electrostatic imagedisp1ay relativley increased conductivity in the such selective field intensification and/or deflection specifically acts to induce increased electroluminescent in portions of the electroluminescent layer adjacent charged areas of the latent image. The electroluminescent in turn impinges upon adjacent areas of the photoconductor surface itself to further reduce its impedance. As will be shown in greater detail in what ensues, the overall reaction is a feed-back phenomena according to which partially discharged areas on the photoreceptor surface become increasingly discharged as contact with the electroluminescent layer is maintained under the influence of suitable potentials between the activating electrodes at the back side of the electroluminescent layer.

A fuller understanding of the present invention and of the manner in which it operates to achieve the objects previously set forth may now best be gained by a reading of the following detailed specification, and by a simultaneous examination of the drawings appended hereto in which:

FIG. 1 graphically depicts the manner in Which the present invention is practiced where initial formation of the faint electrostatic image is on zinc oxide-coated paper.

FIG. 2 shows on a highly magnified scale a portion of the FIG. 1 depiction and illustrates the mechanism involved in the instant invention.

FIG. 3 is a schematic electrical equivalent circuit showing the basic electrical phenomena in FIGS. 1 and 2.

FIG. 4 graphically depicts application of the present method with an alternate electroding scheme.

FIG. 5 graphically depicts another suitable surface upon which the latent electrostatic image may initially be formed; and

FIG. 6 shows a portion of FIG. on a magnified scale and illustrates the mechanism by which conductivity variations arise in the semiconductor layer depicted in FIG. 5.

FIG. 1 graphically depicts a basic technique by which the present invention may be practiced. In that figure an electroluminescent panel 3 is shown having a structure peculiarly adapted for use in practice of the present invention. The panel 3 includes an insulating base 5 which usually will comprise a transparent material such as, for example, glass. An electroluminescent layer 7 adjoins base '5, is of conventional composition and generally comprises an electroluminescent phosphor in an insulating resin binder such as polyvinyl chloride. Typical phosphors utilized include copper chloride-activated zinc sulfide. Or as another typical example of the electroluminescent phosphor, a composition may be utilized containing approximately 80% zinc sulfide and zinc selenide, with copper as an activator. As will be appreciated in the ensuing paragraphs, however, the particular phosphor composition utilized will be chosen with some variation depending upon the wavelength response of the particular photoreceptor utilized in conjunction with the invention. For reasons that will also become apparent in subsequent paragraphs, the thickness of electroluminescent layer 7 is made quite low, usually being of the order of 1 mil or less.

Deposited directly upon the base layer 5 and there fore between the base layer and the adjoining electroluminescent layer 7 are a series of conductive strip electrodes 8. As adjacent strips will ordinarily be operated at opposite potentials, simplicity of wiring may be gained by depositing the electrodes as an interdigitated pattern,

- whereby common connection need be made at only two points in the pattern. The electrodes may by way of example be formed upon the base 7 from evaporated gold or the like, or similarly vacuum deposition techniques may be utilized to lay down apattern of copper or the like.

An essential step in the practice of the present invention involves formation of a latent electrostatic image on a surface contiguous to a semiconductor layer of the type displaying conductivity variations in response to an electric field imposed thereon. For purposes of the present specification a layer of semiconductor material exhibiting this phenomenon will be referred to as a fieldeffect layer in consideration of the use of such semiconductor layers as the essential performing element in the so-called field effect transistors. A large number of semi-conductor materials are known which exhibit the specified characteristics and an extensive list may be found, for example, at page 9 of Field Effect Transistors edited by Wallmark and Johnson, Prentice Hall, Inc., Englewood Cliifs, NI, (1966). In order to form the latent electrostatic image contiguous to such a field-elfect layer the semiconductor material comprising the layer may itself be chosen to possess substantial photoconductivity; or alternatively a highly insulating photoconductive material may be bonded to the semiconductor layer and the latter element may then constitute the situs for the latent electrostatic image.

Of those semiconductor materials which exhibit the required field-effect response and are also substantially photoconductive there exists a subclass of materials particularly suitable for use as the field effect layer of the present invention. This subclass of materials will hereinafter be referred to as storing semiconductors, the term serving to define semiconductor materials adapted to retain electrostatic charge on their surface, to conduct current through the central portion thereof without substantially dissipating such charge and to dissipate such charge in response to impinging radiation. Zinc oxide is the best known example of the materials in the defined subclass; however in addition to zinc oxide there are other materials such as lead oxide and cadmium oxide which exhibit similar characteristics.

In FIG. 1 the photoreceptor 13 is a common sheet of zinc oxide-coated paper and thus includes the paper base 15 and the zinc oxide layer 17. The latter as is well known in the art comprises a zinc oxide pigment dispersed in a relatively transparent binder. Where the photoreceptor 13 is, as indicated, a latent electrostatic image may be formed in the usual manner. This is to say that the photoreceptor 13 may be positioned on a conductive grounded backing 35, the zinc oxide surface may be initially charged to uniform negative potential as from a negative corona source or the like, and thereafter the charged surface may be exposed to a projected optical image. The resulting latent electrostatic image is suggested by the reference numeral 19. It may be further assumed that the optical input supplied to the surface of the illustration is of relatively low intensity so that the elecrostatic laten image 19 is quite faint; in more precise terms this implies that the variation in charge from point to point on the zinc oxide surface is relatively small; that is, the potential variation from point to point is minor in nature.

The object of the present process is now to amplify the relatively slight potential variations present from point to point in the faint latent image so that a more readily utilizable image results. This is brought about in the manner depicted in FIG. 1 which shows the photoreceptor 13 bearing latent image 19 in virtual contact with the outside face of the electroluminescent layer 7. In actual practice the contact between the two surfaces would be real but for purposes of illustration slight separation is depicted in the diagram.

In accordance with the present invention augmentation of the latent image 19 is now achieved by merely activating AC potential source 11. A switch 10- may be provided for such purposes. In the embodiment of FIG. 1 it will be seen that pairs of electrodes of opposite polarities such as 2 and 4 are geometrically positioned with respect to each other, such that in the absence of contact with the photoreceptor 13 the electric field therebetween would be essentially parallel to the lower face of electroluminescent layer 7. In such event little penetration of the electric field into the luminescent layer 7 would occur. In addition the interelectrode capacitance between members such as 2 and 4 is quite low because of the relatively high spacing and the electric field there between is relatively low for a given potential difference. The additive result of these several factors is that little or no luminescence will normally be exhibited between such electrodes. Once the electroluminescent panel 3, however, is positioned adjacent the latent image photoreceptor a new phenomenon occurs. Because the layer 17 is a field-effect layer in the sense that this term has previously been defined, conductivity variations will be exhibited in the volumes of the layer immediately below the latent charge pattern, and these variations will occur in accord with the charge pattern itself. In particular, volumes of relatively decreased conductivity will be present in the zinc oxide layer 17 below areas of relatively high negative charge since such negative charge acts by field effect action to reduce the number of conduction electrons in the body of the zinc oxide semiconductor immediately adjacent the deposited charge. In optical terms the areas previously struck by light will accordingly be relatively conducting in comparison to volumes of the zinc oxide layer below surface areas not struck by lightthat is to say below surface areas still holding substantial quantities of negative charge.

It follows in consequence of the foregoing that the electric fields between the various pairs of adjacent electrodes will be deflected only in those instances where the space between the electrode pair overlies volumes of increased conductivity in the semiconductor layer 17, and that in such instance the deflected field will necessarily pass through adjoining portions of the electroluminescent layer 7. Since the layer 7 is furthermore very thin, the provision of conductive surfaces adjacent thereto provides capacitance transverse to the layer of a magnitude far exceeding the parallel capacitance previously present between the adjacent electrodes themselves. For this reason deflected fields are also greatly intensified ones.

The phenomenon is shown on a magnified scale in FIG. 2 depicting a small section of the FIG. 1 setup. In FIG. 2 an area 23 is shown on the zinc oxide layer 17 relatively free from charge. A second area 25 is also shown containing a relatively high concentration of negative charge. Adjacent the area 25 the volume of zinc oxide has become relatively non-conductive in comparison to the corresponding volume of zinc oxide below the uncharged area 23. The electric field between electrodes 8 and 12 is deflected little if all and this is suggested by the arrow 21 intended to represent the electric field present between these electrodes. On the other hand the field between electrodes 6 and 8, being adjacent to more conductive portions of the zinc oxide layer 17 is deflected in a manner suggested by the arrows at 2,4. The field between electrodes 6 and 8 is thus caused to penetrate through the electroluminescent layer 7; the field is furthermore greatly intensified by the increased capacitance now present across the layer; and electroluminescence develops in this area.

Alternatively one may consider the mechanism by Which increased electroluminescence occurs in layer 7 from a viewpoint of current fiow between adjacent electrodes. Since such electrodes are separated by electroluminescent material it follows that any current flow therebetween necessarily passes through such material, and the electroluminenescence occurring in the material may be considered a function of the amount of current so flowing. In the absence of contact of the electroluminescent layer with the charge-bearing semiconductor, the apparatus equivalent circuit in FIG. 3 would include only the potential source 11, the representative electrodes 6 and 8, and the single conductive path 12 therebetween. The path 12 comprises in series a fixed resistance 18 and a fixed capacitance 10, the latter being indicative of the interelectrode capacitance between electrodes 6 and 8.

Because of the relatively high spacing between such adjacent electrodes, the last named factor-the interelectrode capacitanceis quite small. Once an area of increased conductivity however, is positioned adjacent the electroluminescent layer, a parallel conductive path 14 may be considered to have been established. The current flowing between electrodes 6 and 8 is obviously increased immediately by provision of any parallel path; however it should be noted that since the layer 7 is very thin the new capacitance 16 is far in excess of the capacitance 10. Accordingly the current flowing between the electrode pair is not only increased but increased greatly so, and the increase in electroluminescence may be accordingly attributed principally to the capacitance 16.

The feed-back action of the present process may now be readily understood. Once again examining FIG. 2 it will be appreciated that electroluminescence develops in layer 7 principally in those portions of the layer adjacent areas on the semiconductor field-effect layer which have previously been struck by light; that is to say, areas on the zinc oxide surface showing some degree of discharge. Since the electroluminescent panel 3 and photoreceptor 13 are in virtual contact the luminescence created impinges directly upon such areas of already diminished charge. The luminescence further reduces the charge level, and thus begins a positive feedback cycle serving to reduce lower and lower the charge levels in those areas originally exhibiting diminished charge.

Once the feed-back cycle has progressed sufliciently to provide the desired degree of enhancement it is only necessary to open the switch 10, or alternatively remove the member 3 from its adjacency to the charge image 19. While the ideal time to produce a particular enhancement may in the first instance have to be experimentally determined for any particular photoreceptor, automated methods may thereafter be used. Aside from the obvious expedient of controlling the exposure time, one may for example monitor the current in ground lead 33, for the latter will rise as the contact exposure progressively increases the conductivity in layer 17. Once such current reaches a predetermined value therefore a reliable indicator is present that sufficient enhancement exists, and switch 10 may be opened.

In FIG. 4 a modification of the FIG. 1 setup is shown which illustrates a quite different electroding arrangement for providing the conditions under which the present invention may be practiced. Once again in FIG. 4 it is assumed that a latent electrostatic charge image is formed upon the surface of the photoreceptor 13, which as in FIG. 1 includes a field-effect semiconductor layer 17 of the storing semiconductor variety, which may specifically comprise a zinc oxide coating of the order of 1 mil or less thickness. In the present instance, the zinc oxide is not upon a paper-like base but rather is directly afiixed to a conductive substrate 36. The latter may be of any convenient thickness; however, it is convenient to consider conductive substrate 36 as constituting a thin conductive foil of aluminum or the like. A commercial product including a zinc oxide layer backed by such a conductive foil is in fact available commercially from the 3M Company of Minneapolis, Minn. It may be assumed that the latent charge image has previously been formed upon the surface 37 of the zinc oxide layer 17, after which the member 13 is brought into contact with the electroluminescent panel 38. The latter includes an insulating support base 39, which may be glass, upon which is coated a thin conductive layer 40 such as of tin oxide. The combination of a glass support base with such a conductive tin oxide coating is in fact available commercially under the trade name NESA from the Corning Glass Works, Corning, NY. Directly deposited upon the substrate 39 and conductive coating 40 is a thin layer 45 of electroluminescent material, usually of the order of 1 mil or so, which constitutionally is similar to the layer 7 described in connection with FIG. 1. An AC potential source 11 is connected through a switch 10 to the electrodes provided by conductive base 36 and conductive coating 40.

In order to produce latent image intensification in accordance with the techniques which have previously been set forth, the sandwich structure shown in FIG. 4 is established, after which switch 10 is closed for a time period appropriate for the degree of enhancement desired. Unlike the FIG. 1 embodiment, it will be appreciated that the electroding arrangement in the FIG. 4 showing is such as to at all times provide an electric field directly transverse to the sandwich structure shown. In terms of electric fields therefore, one may consider the primary physical mechanism producing the image intensification to be one of selective field intensification across those portions of the sandwich which include volumes of material within the zinc oxide layer 17 which display conductivity. Such selective field intensification produces selective electroluminescence in the layer 45, which in turn acts in the same feedback manner as has been previously described in connection with FIG. 1 to augment the charge variations present at surface 37.

The present inventive method is particularly adapted for use with the zinch oxide photoreceptor described in connection with FIG. 1. This is indeed a most fortunate result in view of the fact that zinc oxide coated papers are among the commonest, cheapest, and most efiec tive xerographic materials, and in view further of the fact that these same zinc oxide materials display somewhat lower photosensitivity than other common xerographic receptors such as selenium and are therefore just those materials which are most in need of latent image intensification. However the present invention is no way limited to utilization with zinc oxide or similar storing semiconductors but may be utilized in other invironments, providing only that the initial step in practice of the process involves formation of the latent electrostatic image on a photoconductive surface contiguous to a field-elTect layer. To illustrate this point a depiction of another type of photoreceptor is shown in FIG. 5, which receptor may be similarly utilized in the present invention.

In FIG. a photoreceptor 13 is shown which now consists of two distinct layers plus a conductive support layer 30. 31 is the field-effect layer corresponding partly in function to the zinc oxide layer 17 of FIG. 1. However in the present instance this field-effect layer 31 is not a storing semiconductor as is the ease with zinc oxide but rather a more general case is considered wherein the semiconductor material comprising layer 31 is neither an excellent photoconductor nor displays the ability to retain charge on its surface directly for sustained periods of time. In a typical instance layer 31 would by way of example comprise a vacuum-deposited layer of cadmium sulfide of the order of microns or so thickness. The photoconductive surface contiguous to the semiconductor now takes the form of a separate layer 32 of vitreous selenium. An additional thin insulating interface layer may also be used between layers 31 and 32 to discourage charge injection from the photoconductor to the semiconductor, however this is not always necessary.

In use in the present invention the photoreceptor 13 is positioned in contact with a ground plane 35 and is initially charged by a corona source or the like to a uniform potential. In the present instance a uniform negative potential is utilized for reasons that will become apparent momentarily. The uniformly charged surface of selenium layer 32 is therafter exposed to an optical image to form the latent electrostatic image 19. Again here for purposes of illustrating operation of the present process it may be assumed that the exposure is of relatively low light intensity with the resulting latent image 19 being therefore relatively faint; which is to say that the potential variations induced by the light exposure from point to point on the charged surface are of relatively small magnitude.

FIG. 6 depicts in detail a small section of the FIG. 5 showing and is intended to detail the mechanism by which conductivity variations are caused in the semiconductor field-effect layer 31 by exposure of the uniformly charged selenium surface to light. In this depiction we assume that light has struck the surface of layer 32 approximately in the area designated at 42, while the area at 41 has remined in darkness. In the latter area the charge pattern designated by the minus signs has accordingly remained essentially unaltered, whereas in the former area the selenium has been rendered conductive with the result that charge has been partially dissipated at such points. Assuming that the semiconductor layer 31 constitutes as has previously been suggestedan n-type semiconductor such as cadmium sulfide, the conductivity in volumes of the semiconductor below area 42 such as 44 will be increased by virtue of the decreased electric field. At this point in time the weak electrostatic image may be placed into contact with the electroluminescent panel 3 and the precise procedure followed as was described in connection with FIGS. 1 and 2. Since increased conductivity is present in those areas which have been partially discharged by the action of the light input image, activation of the potential source 11 (FIG. 1) will produce selective luminescence in those parts of the panel adjacent to volumes of increased conductivity-such as 44 in FIG. 4. A positive feed-back action is thus once again set up which serves to increasingly discharge those areas where initial charge dissipation has already occurred, with resulting intensification of charge differences between varying portions of the latent electrostatic image.

The present invention has thus far been principally described in connection with embodiments suitable for image enhancement. This is because in the most frequent caseespecially where photoconductors are involvedthe problem is more often one of low gamma characteristics, rather than of too'high gamma response. However, it will be appreciated that the same technique may be utilized with but simple modifications to produce a degenerative or negative feedback to the latent image so that less rather than more variation is present in charge density between portions of the photoconductor struck and not struck by light. Basically all that is necessary to achieve such a result is to utilize a proper type semiconductor and charge polarity so that dissipation of charge lessens conductivity in the semiconductor rather than increases it. Such a result for example may be brought about in FIG. 4 by merely utilizing an initially positive charge on surface 37, rather than the negative charge shown.

While the present invention has been particularly described in connection with specific embodiments thereof it will be understood by those skilled in the art that in view of the instant disclosure numerous modifications thereof and deviations therefrom may be readily devised without yet departing from the present teaching. Accordingly the instant invention to be broadly construed and limited only by the scope and spirit of the c aims now appended hereto.

What is claimed is:

l. A method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:

(a) forming said latent electrostatic image on a photoconductive surface contiguous to a semiconductor layer, said semiconductor layer being adapted to vary in conductivity in response to the electric field imposed by adjacent portions of said image;

(b) forming a sandwich structure by contacting said image on said photoconductive surface with an electroluminescent layer; and

(c) providing an electrical potential across said sandwich structure whereby an electric field appears at said electroluminescent layer having an intensity at points of said layer in accord with the conductivity of adjacent portions of said semiconductor layer, whereby luminescence occurs in said electroluminescent layer in accord with said conductivity pattern and said electroluminescent layer having an intensity at by feed-back action to amplify charge variations in said image.

2. A method according to claim 1 wherein said semiconductor layer is a storing semiconductor whereby said photoconductor surface bounds said semiconductor layer.

3. A method according to claim 2 wherein said storing semiconductor comprises zinc oxide in a binder.

4. A method according to claim 1 wherein said photoconductive surface comprises the non-adjacent face of a selenium layer positioned adjacent said semiconductor layer.

5. A method according to claim 1 wherein said electrical potential is provided by electrodes at the outside of said sandwich.

6. A method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:

(a) forming said latent electrostatic image on a surface of a photoconductive, field-effect semiconductor layer, said field-effect semiconductor exhibiting field-effect properties such that the conductivity of portions thereof will vary in accordance with the configuration of said latent electrostatic charge image formed thereon;

(b) forming a sandwich structure by at least virtually contacting said latent electrostatic charge image-retaining semiconductor surface with an amplifying element, said amplifying element comprising a supporting substrate, a plurality of interdigital electrodes supported by said substrate, alternating electrodes being connected to one side of an alternating current potential source with the intermediate electrodes being connected to the other side of said potential source, and a layer of electroluminescent phosphor material overlying said plurality of electrodes, said electroluminescent layer being adjacent said latent electrostatic charge image-retaining semiconductor surface; and

(c) applying an electrical potential across said sandwich structure by means of said alternating current potential source, the electrical field established thereby being deflected through at least a portion of said electroluminescent layer beneath areas of said latent electrostatic charge image of reduced charge intensity, whereby electroluminescence occurs in those portions of said electroluminescent layer through which said field is deflected, said luminescence serving by feed-back action to discharge adjacent portions of said photoconductive, field-effect semiconductor layer thereby amplifying charge variations in said latent electrostatic charge image thereon.

7. The method of claim 6 wherein said semiconductor layer comprises zinc oxide in an insulating binder.

8. A method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:

(a) forming said latent electrostatic image on a surface of a photoconductive insulating material;

(b) forming a sandwich structure by at least virtually contacting said latent electrostatic charge image-retaining photoconductive insulating material surface with an amplifying element while the opposed surface of said photoconductive insulating material is in contact with a field-effect semiconductor layer, said field-effect semiconductor exhibiting field-effect properties such that the conductivity of portions thereof will vary in accordance with the configuration of said latent electrostatic charge image formed on said photoconductive insulating material surface, said amplifying element comprising a supporting substrate, a plurality of interdigital electrodes supported by said substrate, alternating electrodes being connected to one side of said alternating current potential source with the intermediate electrodes being connected to the other side of said potential source, and a layer of electroluminescent phosphor material overlying said plurality of electrodes, said electroluminescent layer being adjacent said latent electrostatic charge image-retaining photoconductive insulating material surface; and

(c) applying an electrical potential across said sandwich structure by means of said alternating current potential source, the electric field established thereby being deflected through at least a portion of said electroluminescent layer beneath areas of said latent electrostatic charge image of reduced charge intensity, whereby luminescence occurs in those portions of said electroluminescent layer through which said field is deflected, said luminescence serving by feedback action to discharge adjacent portions of said photoconductive insulating layer thereby amplifying charge variations in said latent electrostatic charge image thereon.

9. The method of claim 8 wherein said photoconductive insulating material comprises selenium.

10. The method of claim 9 wherein said field-effect semiconductor is cadmium sulfide.

11. A method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:

(a) forming said latent electrostatic image on a surface of a photoconductive, field-effect semiconductor layer, said field-effect semiconductor exhibiting fieldeifect properties such that the conductivity of portions thereof will vary in accordance with the configuration of said latent electrostatic charge image formed thereon;

(b) forming a sandwich structure by at least virtually contacting said latent electrostatic charge image-retaining semiconductor surface with an amplifying element while the opposed surface of said semiconductor layer is in contact with a conductive backing member, said amplifying element comprising a supporting substrate, a conductive layer overlying said supporting substrate and a layer of electroluminescent phosphor material overlying said conductive layer, said electroluminescent layer being adjacent said latent electrostatic charge image-retaining semiconductor surface, said conductive backing member and said conductive layer being connected to opposite sides of an alternating current potential source; and

(c) applying an electrical potential across said sandwich structure by means of said alternating current potential source, the electrical field established thereby being selectively intensified through those portions of said electroluminescent layer beneath areas of said latent electrostatic charge image of reduced charge intensity, whereby luminescence occurs in those portions of said electroluminescent layer through which said field is selectively intensified, said luminescence serving by feed-back action to discharge adjacent portions of said photoconductive, field-effect semiconductor layer thereby amplifying charge variations in said latent electrostatic charge image thereon.

12. The method of claim 11 wherein said semiconductor layer comprises zinc oxide in an insulating binder.

13. A method for amplifying charge variations in a (a) forming said latent electrostatic image on a surface of a photoconductive insulating material;

(b) forming a sandwich structure by at least virtually contacting said latent electrostatic charge imageretaining photoconductive insulating material surface with an amplifying element while the opposed surface of said photoconductive insulating material is in contact with a field-effect semiconductor layer sandwiched between said photoconductive insulating material and a conductive backing member, said fieldeifect semiconductor exhibiting field-effect properties such that the conductivity of portions thereof will vary in accordance with the configuration of said latent electrostatic charge image formed on said photoconductive insulating material surface, said amplifying element comprising a supporting substrate, a conductive layer overlying said supporting substrate and a layer of electroluminescent phosphor material overlying said conductive layer, said electroluminescent layer being adjacent said latent electrostatic charge image-retaining photoconductive insulating material surface, said conductive backing member and said conductive layer being connected to opposite sides of an alternating current potential source; and

(c) applying an electrical potential across said sandwich structure by means of said alternating current potential source, the electrical field established thereby being selectively intensified through those portions of said electroluminescent layer beneath areas of said latent electrostatic charge image of reduced charge intensity, whereby luminescence occurs in those portions of said electroluminescent layer through which said field is selectively intensified, said luminescence 11 12 serving by feed-back action to discharge adjacent por- 3,169,192 2/1965 Kohashi 2502l3 tions of said photoconductive insulating material 3 186 839 6/1965 Stone et 1. 25() 213 X thereby amplifying charge variations in said latent 3,264,479 8/1966 p 250*213 electrostatic charge image thereon. 3,322,539 5/1967 Redington 355 17 X l method of Claim 13 wherein t Photocon" 5 3,348,074 10/1967 Diemer 317 235 X ductive lnsulating material comprises selenium. 3 441 736 4/1969 v 15. The method of claim 14 wherein said field-effect Kazan et 250 213 t fid Semlconduc of 1s Cadmlum sul e WALTER STOLWEIN, Primary Examiner References Cited UNITED STATES PATENTS 2,927,234 3/1960 Kazan 250213 X 10 US. 01. X.R. 

